The proof comes in the form of P/2013 R3, discovered in September 2013. When it was first seen by the Catalina Sky Survey and Pan-STARRS (which sweep the sky looking for moving objects) and then again by the huge Keck telescope in Hawaii, it appeared to be three co-moving objects surround by a cloud of material.

The images show the collection of rocks over time, and you can see them change position. Some of that is due to real motion, and some due to the changing perspective in which we see them as both the Earth and the swarm orbit the Sun. At least 10 objects are seen, the biggest of which are about 200 meters across. All of them have tails that look suspiciously like comets … but clearly aren’t.

We know this because the object(s) orbit the Sun in the main asteroid belt between Mars and Jupiter. Comets don’t usually have orbits like that. Also, a comet on that orbit would have long since lost all its volatile material (stuff like ice that can be vaporized by sunlight and expand away as a gas). The event we’re seeing here can’t have started too long ago, and we’ve caught it while it’s still happening.

The first thing you might think of is an impact by another asteroid. We’ve seen that before. But in this case the debris isn’t flying apart quickly enough; they’re moving at a glacial 1.5 kilometers per hour—slower than walking speed! An impact strong enough to disrupt an asteroid should have much faster-moving shrapnel than that. Also, tracing the motions back, the pieces didn’t all fly off at the same time, so it wasn’t a single, catastrophic event that started them on their way.

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More likely, what we’re seeing here is the final stages of the YORP effect. This is an incredible slow and gentle process where sunlight heats up one side of a rotating, lumpy object. The object re-radiates that heat away as infrared light, which has a teeny tiny amount of momentum. Although this is a quantum effect, and very small, over time this can act to speed up the rotation of the rock and get it spinning at a substantial clip. We’ve actually seen the effects of this before, on an asteroid called P/2010 A2 (LINEAR) and another recently observed one called P/2013 P5, which sprouted six different comet-like tails! (Note the designation P for all these objects; that stands for “periodic comet,” because the naming convention dictates that anything that sends out material like this has cometary properties; I think this makes sense up to a point but is confusing in the long run. But then, comets and asteroids are very similar, so it’s the nature of nature to blur the lines between categories.)

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

In those other cases, what probably happened is that loose material on the surfaces of those objects got flung off when the parent body started spinning so rapidly that the centrifugal force overcame that of gravity. In this case, though, the main body of the asteroid itself broke apart!

This means the parent body couldn’t have been a solid rock, or else it would have been a lot sturdier. Instead, it was likely what astronomers call a rubble pile. Over time, a solid asteroid suffers myriads of minor collisions with smaller asteroids. These may not be sufficient to destroy it, but they may be enough to fragment it, creating deep cracks throughout. Instead of a solid piece, the asteroid is more like a big bag of broken glass, held together only by the cumulative gravity of all the pieces.

But at some point, the YORP effect spins it up enough that the meager gravity of the small component rocks isn’t enough to keep it together. The individual pieces eventually move apart. The expected velocity would be low, since the YORP process is gradual, and when it finally overcomes gravity the speed of the pieces won’t be terribly fast.

And that’s just what we’re seeing here, along with general detritus and debris flying off as well (probably stuff that was between the other pieces, inside the original asteroid). As far as I can tell, this is the very first time we’ve seen the YORP effect totally disrupt an asteroid, which is pretty dang cool. In a sense, it’s proof that some asteroids really are just big rubble piles, as we expected.

Asteroids are amazing. If I had to do it all over again and start my career over from scratch researching some other topic in astronomy, it would either be these guys or exoplanets. But the beauty of my career now is that I get to read all about everything. And then I also get to pass it on to you, so you can see what I see: just how wonderful and surprising and simply downright cool the Universe really is.